In recent years, exploration and production of hydrocarbons has extended such operations to Arctic, sub-Arctic, and other ice-prone offshore environments where large bodies of moving ice are found. These large moving bodies of ice can severely damage offshore exploration, development, or production vessels, such as offshore drilling units (“MODU”), platform vessels, or jackups.
An example of such an area is off the north coast of Alaska in the Beaufort Sea. With the onset of winter, the sea water near the coastline begins to freeze over. The freeze over results in the formation of a relatively smooth and continuous sheet of ice called “fast ice” which extends seaward from the shore to points which lie over water approximately 60 feet deep. The name fast ice implies that this sheet of ice is held fast to the land and does not move. However, fast ice can be moved by natural forces, such as currents, tides, and temperature changes, with the rate of movement being generally dependent on the thickness of the ice.
When set in motion, fast ice poses a threat to offshore operations. When the ice comes into direct contact with an offshore drilling structure, such as a production platform, large forces can develop. These forces cause the ice sheet to break and pile up directly against the offshore structure, forming a rubble field. As the rubble field grows and continues to be pressed against the structure, the forces can increase until the structure is seriously damaged.
Although it is subject to movement, fast ice is relatively stable during the winter. However, the fast ice sheet breaks up during the summer, resulting in the formation of many individual floating bodies of ice which are free to move about under the influence of winds and currents. These moving bodies of ice pose another threat to offshore operations.
Seaward of the fast ice zone is pack ice. Unlike fast ice, pack ice is discontinuous, rugged, and highly mobile. As pack ice moves, local areas of tension and compression develop, causing the ice to break and pile up. As a result, open leads and pressure ridges are formed.
Pressure ridges form in areas of pack ice which experience large compressive forces. The ice breaks and piles up, concentrating large masses of ice into relatively small areas. Pressure ridges extend well above and below the surrounding ice, and some are so large that they are able to survive the summer and become multi-year ice features.
During the winter season, many pressure ridges are embedded in the pack ice and move along with it, threatening any structure in their path. During the summer, pressure ridges can be blown toward shore, where they threaten structures and vessels which lie in shallow waters.
Heretofore, strategies for Arctic exploration, development, and production have included the construction of new-build, ice capable vessels and the reinforcement of existing vessels to make them ice capable. However, these approaches may impose prohibitively high costs and/or prohibitively long timelines that are inconsistent with the desire for quick deployment in Arctic exploration, development, or production operations during times of favorable business environments.